1. Field of the Invention
The present invention relates generally to an exhaust emission control system of an internal combustion engine and, more particularly, to an exhaust emission control system of an internal combustion engine which is capable of switching over a flow direction of an exhaust gas flowing through an exhaust gas purifying element according to the necessity.
2. Related Background Art
In general, an exhaust emission control system for purifying an exhaust gas discharged from an internal combustion engine is provided at an exhaust passageway of the internal combustion engine. When the exhaust gas from the internal combustion engine flows through this exhaust emission control system, a deposit is gradually adhered from an upstream side in the exhaust emission control system. A classification of what this deposit is all about might differ depending upon a composition of the exhaust gas, or a construction of the exhaust emission control system or an exhaust gas purifying mechanism, and what can be exemplified as the deposit may be, e.g., an oxide, a sulfide, nitrate and sulfate. This deposit might cause a decline of a purging performance of the exhaust emission control system and also an increase in an exhaust resistance, and therefore needs to be removed at a predetermined timing.
For example, the exhaust emission control system for purging the exhaust gas of NOx which is discharged from the internal combustion engine for performing a combustion at a lean air/fuel ratio, may involve the use of an NOx storage-reduction catalyst. This NOx storage-reduction catalyst absorbs NOx when the air/fuel ratio of the inflow exhaust gas is lean, and desorbs NOx absorbed thereto when a concentration of oxygen in the inflow exhaust gas decreases, thus effecting reduction to N2. The NOx storage-reduction catalyst is disposed in an exhaust passageway, and absorbs a nitrogen oxide (NOx) contained in the exhaust gas exhibiting the lean air/fuel ratio. After absorbing NOx, the air/fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is made rich by increasing a quantity of the fuel supplied to the internal combustion engine, thereby desorbing NOx absorbed thereto from the NOx storage-reduction catalyst. Desorbed NOx is reduced to N2 with a reducing component such as unburned HC, CO etc contained in the exhaust gas.
By the way, generally speaking, the fuel of the internal combustion engine contains a sulfur content, and, when the fuel is burned in the internal combustion engine, the sulfur content is burned, resulting in a production of sulfur oxide (SOx). The NOx storage-reduction catalyst absorbs SOX contained in the exhaust gas with the same mechanism as absorbing NOx. Therefore, if the NOx storage-reduction catalyst is disposed in the exhaust passageway of the internal combustion engine, the NOx storage-reduction catalyst absorbs SOx as well as NOx.
SOx absorbed to the NOx storage-reduction catalyst, however, generates stable sulfate with a passage of time. SOx is therefore dissolved and desorbed with a difficulty and has a tendency of being easily accumulated within the NOx storage-reduction catalyst under a condition of executing desorption, reduction and purging of NOx out of the NOx storage-reduction catalyst (which is hereinafter termed a NOx desorbing/reducing process). If there augments a SOx accumulation quantity within the NOx storage-reduction catalyst, a NOx absorption quantity of the NOx storage-reduction catalyst decreases, and it is therefore unfeasible to sufficiently purge the exhaust gas of NOx, with the result that so-called SOx poisoning occurs, wherein a NOx, purging rate declines. Such being the case, it is required that SOx absorbed to the catalyst be desorbed therefrom at a proper timing in order to keep high the NOx purging rate of the NOx storage-reduction catalyst for a long period of time.
It has already proved that the air/fuel ratio of the inflow exhaust gas needs to be rich and the NOx storage-reduction catalyst is required to be set at a higher temperature than in the NOx desorbing/reducing process for desorbing SOx absorbed to the NOx storage-reduction catalyst.
Incidentally, a distribution of a SOx absorption quantity in the NOx storage-reduction catalyst exhibits a higher concentration in the closer proximity to the inlet of the exhaust gas in the NOx storage-reduction catalyst. Hence, when desorbing SOx absorbed to the NOx storage-reduction catalyst, in the case of flowing the exhaust gas having the rich air/fuel ratio in the same direction as a flow direction of the exhaust gas when absorbing NOx, though SOx absorbed is desorbed on the inlet side in the NOx storage-reduction catalyst, SOx desorbed therefrom merely migrates to the outlet side of the exhaust gas through the NOx storage-reduction catalyst and is reabsorbed to the NOx storage-reduction catalyst. The problem is therefore such that SOx can not be discharged from the NOx storage-reduction catalyst.
Under such circumstances, a technology disclosed in Japanese Patent Application Laid-Open Publication No.7-259542, is that when desorbing SOx absorbed to the NOx storage-reduction catalyst, the exhaust gas having the rich air/fuel ratio flows through the NOx storage-reduction catalyst in a direction opposite to the direction when absorbing NOx. In the case of incorporating a backward flow function of desorbing SOx by reversing the flow of the exhaust gas as described above, SOx desorbed from the NOx storage-reduction catalyst has a shorter migration distance within the NOx storage-reduction catalyst, and is immediately discharged out of the NOx storage-reduction catalyst. It is therefore feasible to prevent desorbed SOx from being reabsorbed to the NOx storage-reduction catalyst.
The following is an explanation of a construction of the exhaust mission control system of the internal combustion engine which incorporates the backward flow function disclosed in the above Publication. An upstream-side exhaust passageway connected to an inlet of the NOx storage-reduction catalyst is connected to a downstream-side exhaust gas passageway connected to an outlet of the NOx storage-reduction catalyst via a bypass passageway for bypassing the NOx storage-reduction catalyst. A first emission flow switching valve is provided at a confluent portion between the upstream-side exhaust passageway and the bypass passageway. A second emission flow switching valve is provided at a confluent portion between the downstream-side exhaust passageway and the bypass passageway. The first emission flow switching valve is capable of making a changeover to let the exhaust gas flowing from upstream flow through the NOx storage-reduction catalyst or let the exhaust gas flow into the bypass passageway. The second emission flow switching valve is capable of making a changeover to let the exhaust gas flowing through the NOx storage-reduction catalyst flow out toward the downstream-side exhaust passageway disposed more downstream than the second emission flow switching valve or let the exhaust gas flowing though the bypass passageway flow out toward the downstream-side exhaust passageway disposed more downstream than the second emission flow switching valve. Further, an exhaust passageway for suction is bypassed from the upstream-side exhaust passageway between the NOx storage-reduction catalyst and the first emission flow switching valve, and is connected to an intake port of an exhaust pump, and a discharge port of the exhaust pump is connected to the above bypass passageway. Moreover, a reducing agent supply device for supplying a reducing agent is provided at the downstream-side exhaust passageway between the NOx storage-reduction catalyst and the second emission flow switching valve.
Then, when in the NOx absorbing process, the first and second emission flow switching valves are switched over to close the bypass passageway so that an entire quantity of exhaust gas from the internal combustion engine flows toward the outlet from the inlet of the NOx storage-reduction catalyst. On the other hand, when desorbing SOx from the NOx storage-reduction catalyst, the first and second emission flow switching valves are switched over to close the bypass passageway so that substantially the entire quantity of exhaust gas from the internal combustion engine flows to the bypass passageway. Simultaneously, the exhaust gas in the upstream-side exhaust passageway between the NOx storage-reduction catalyst and the first emission flow switching valve is sucked and discharged to the bypass passageway by operating the exhaust pump, thereby causing a flow of the exhaust gas flowing backward from the outlet toward the inlet through the NOx storage-reduction catalyst. Besides, the reducing agent is supplied to the downstream-side exhaust passageway by operating a reducing agent supply device. The exhaust gas exhibiting the rich air/fuel ratio is thereby flowed backward through the NOx storage-reduction catalyst, thus desorbing SOx from the NOx storage-reduction catalyst.
The conventional backward-flow-function-incorporated exhaust emission control system of the internal combustion engine requires the exhaust pump and the plurality of emission flow switching valves and therefore involves the use of an increased number of parts, resulting in a rise in costs. Further, the increased number of parts leads to a good deal of labors for maintenance and inspection, correspondingly.
Moreover, the SOx desorbing process involving the backward flow in the NOx storage-reduction catalyst may be defined as a processing method giving attention to avoidance of the SOx reabsorption by decreasing the SOx migration distance when desorbing SOx. While on the other hand, if this processing method is adopted, there increases a distance till the exhaust gas arrives at the NOx storage-reduction catalyst, and hence there must be a large drop in temperature of the exhaust gas during a period for which the exhaust gas flows through this long route. The above processing method is not necessarily, as the case may be, considered the best method of desorbing SOx in terms of a temperature condition when desorbing SOx.